The slots for global settings are quite well-described in the manual. See GLOBAL SETTINGS. Each slot contains settings for Control, Routing, and Sync. In your band analogy, you might use different global slots for when the band is in the studio or performing live (because you might be, for example, slaving to a DAW in the studio, or hardware only when playing live, or using all the individual outs in the studio or just a stereo out for live).
Your example of having different sample sets for two different bands is more like having two different snapshots (one for each band) on the + Drive. There are eight global settings slots available in each snapshot.